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| United States Patent |
6,178,074
|
|
Gill
|
January 23, 2001
|
Double tunnel junction with magnetoresistance enhancement layer
Abstract
An apparatus and method is disclosed for an enhanced double tunnel junction
sensor which utilizes an enhancement layer(s) to enhance magnetoresistance
(MR coefficient) and resonant tunneling. Additionally, a combined
read/write head and disk drive system is disclosed utilizing the enhanced
double tunnel junction sensor of the present invention. The enhancement
layers improve the resonant tunneling and boost the MR coefficient to
achieve a higher tunnel magnetoresistance (TMR) for the structure with
applied dc bias. This is accomplished by using enhancement layers that
create a quantum well between the enhancement layer and the pinned layer,
which causes resonance, enhancing the tunneling electrons. By doing this,
the tunneling constraints on the free layer are decoupled, allowing the
free layer to be made thicker which results in reducing or eliminating
free layer magnetic saturation caused by an external magnetic source. As
the enhanced double tunnel junction sensor is positioned over the magnetic
disk, the external magnetic fields sensed from the rotating disk moves the
direction of magnetic moment of the free layer up or down, changing the
resistance through the tunnel junction sensor. As the tunnel current is
conducted through the tunnel junction sensor, the increase and decrease of
electron tunneling (i.e., increase and decrease in resistance) are
manifested as potential changes. These potential changes are then
processed as readback signals by the processing circuitry of the disk
drive.
| Inventors:
|
Gill; Hardayal Singh (Portola Valley, CA)
|
| Assignee:
|
International Business Machines Corporation (Armonk, NY)
|
| Appl. No.:
|
196446 |
| Filed:
|
November 19, 1998 |
| Current U.S. Class: |
360/324.2 |
| Intern'l Class: |
G11B 005/39 |
| Field of Search: |
360/313,314,315,324,324.2
257/421,422
365/158
324/252
338/32 R
|
References Cited
U.S. Patent Documents
| 5390061 | Feb., 1995 | Ryoichi et al. | 360/324.
|
| 5629922 | May., 1997 | Moodera et al. | 369/126.
|
| 5636093 | Jun., 1997 | Gijs et al. | 360/126.
|
| 5650958 | Jul., 1997 | Gallagher et al. | 365/173.
|
| 5654566 | Aug., 1997 | Johnson | 257/295.
|
| 5708358 | Jan., 1998 | Ravipati | 324/252.
|
| 5715121 | Feb., 1998 | Sakakima et al. | 360/324.
|
| 5747859 | May., 1998 | Mizushima et al. | 360/324.
|
| 5898548 | Apr., 1999 | Dill et al. | 360/324.
|
| 5936293 | Aug., 1999 | Parkin | 257/422.
|
| 5966275 | Oct., 1999 | Iijima | 360/324.
|
| 5986858 | Nov., 1999 | Sato et al. | 360/324.
|
| 6005753 | Dec., 1999 | Fontana, Jr. et al. | 360/324.
|
| Foreign Patent Documents |
| 7-210832 | Aug., 1995 | JP.
| |
| 10-91925 | Apr., 1998 | JP.
| |
Other References
"Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film
Tunnel Junctions", Physical Review Letters, vol.74, No. 16, Apr. 17, 1995.
"Spin-Polarized Transport", Physics Today, Prinz, Gary A., Apr. 1995.
|
Primary Examiner: Ometz; David L.
Attorney, Agent or Firm: Gray Cary Ware & Freidenrich LLP, Johnston; Ervin F.
Claims
What is claimed is:
1. A resonant enhanced double tunnel junction sensor that has an air
bearing surface (ABS) comprising:
an electrically conductive first shield layer;
an electrically conductive first antiferromagnetic pinning layer that has a
magnetic moment oriented in a first predetermined direction;
an electrically conductive first ferromagnetic pinned layer exchange
coupled to the first pinning layer so that a magnetic moment of the first
pinned layer is pinned in the first predetermined direction;
a first enhancement layer capable of enhancing spin polarized resonant
tunneling;
a first spacer layer;
a ferromagnetic free layer that has a magnetic moment in a second
predetermined direction, the second predetermined direction being
different than the first direction, the magnetic moment being free to
rotate relative to the second predetermined direction in response to an
applied magnetic field;
a second spacer layer;
a second enhancement layer capable of enhancing spin polarized resonant
tunneling;
an electrically conductive second ferromagnetic pinned layer;
an electrically conductive second antiferromagnetic pinning layer that has
magnetic spins oriented in the first predetermined direction and by
exchange coupling pins the magnetic moment of the second pinned layer in
the first predetermined direction; and
an electrically conductive second shield layer.
2. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the first and second shield layers are used for electrical leads.
3. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the electrically conductive first and second antiferromagnetic
pinning layers have a thickness from 50 .ANG. to 250 .ANG..
4. A resonant enhanced double tunnel junction sensor as claimed in claim
wherein the conductive antiferromagnetic pinning layers are selected from
the group PtMn, MnFe, NiMn, and IrMn.
5. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the electrically conductive first and second ferromagnetic pinned
layers have a thickness from 20 .ANG. to 60 .ANG..
6. A resonant enhanced double tunnel junction sensor as claimed in claim
wherein the ferromagnetic pinned layers are selected from the group
consisting of Co.sub.90 Fe.sub.10, NiFe and Co.
7. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the first ferromagnetic pinned layer is an antiparallel (AP)
pinned layer that includes:
a ruthenium (Ru) film layer and first and second ferromagnetic pinned film
layers;
the ruthenium layer being located between the first pinned film layer and
the second pinned film layer;
the first pinned film layer exchange coupled to the first pinning layer so
that a magnetic moment of the first pinned film layer is pinned in the
first predetermined direction; and
the second pinned film layer having a magnetic moment in a third direction,
the third direction being antiparallel to the first direction.
8. A resonant enhanced double tunnel junction sensor as claimed in claim 7
wherein the first and second pinned film layers are made from cobalt (Co).
9. A resonant enhanced double tunnel junction sensor as claimed in claim 8
wherein the first and second pinned film layers have a thickness of 25
.ANG. and 20 .ANG. respectively and the ruthenium layer has a thickness of
8 .ANG..
10. A resonant enhanced double tunnel junction sensor as claimed in claim 7
wherein the second ferromagnetic pinned layer is an antiparallel (AP)
pinned layer that includes:
a ruthenium (Ru) film layer and third and forth ferromagnetic pinned film
layers;
the ruthenium layer being located between the third pinned film layer and
the forth pinned film layer;
the forth pinned film layer exchange coupled to the second pinning layer so
that a magnetic moment of the forth pinned film layer is pinned in the
first predetermined direction; and
the third pinned film layer having a magnetic moment in a third direction,
the third direction being antiparallel to the first direction.
11. A resonant enhanced double tunnel junction sensor as claimed in claim
10 wherein the third and forth pinned film layers are made from cobalt
(Co).
12. A resonant enhanced double tunnel junction sensor as claimed in claim
11 wherein the third and forth pinned film layers have a thickness of 20
.ANG. and 25 .ANG. respectively and the ruthenium layer has a thickness of
8 .ANG..
13. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the first and second enhancement layers have a thickness of 10
.ANG..
14. A resonant enhanced double tunnel junction sensor as claimed in claim
13 wherein the enhancement layers are made from Cu or Al.
15. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the first and second spacer layers have a thickness from 10 .ANG.
to 30 .ANG..
16. A resonant enhanced double tunnel junction sensor as claimed in claim
15 wherein the spacer layers are made from aluminum oxide.
17. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the ferromagnetic alloy free layer has a thickness from 30 .ANG.
to 100 .ANG..
18. A resonant enhanced double tunnel junction sensor as claimed in claim
17 wherein the ferromagnetic alloy free layer is made from NiFe.
19. A resonant enhanced double tunnel junction sensor as claimed in claim
17 wherein the ferromagnetic alloy free layer is made from a combination
of 25% Co.sub.90 Fe.sub.10 and 75% NiFe.
20. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the first and second shields are made from Permalloy (Ni.sub.80
Fe.sub.20).
21. A resonant enhanced double tunnel junction sensor as claimed in claim 1
wherein the first predetermined direction being normal to the ABS.
22. A magnetic head that has an air bearing surface (ABS) comprising:
a read head that includes:
a resonant enhanced double tunnel junction sensor responsive to applied
magnetic fields;
first and second electrically conductive lead layers connected to the
double tunnel junction sensor for conducting a tunnel current through the
double tunnel junction sensor; the double tunnel junction sensor
including:
an electrically conductive first shield layer;
an electrically conductive first antiferromagnetic pinning layer that has
magnetic spins oriented in a first predetermined direction;
an electrically conductive first ferromagnetic pinned layer exchange
coupled to the first pinning layer so that a magnetic moment of the first
pinned layer is pinned in the first predetermined direction;
a first enhancement layer capable of enhancing spin polarized resonant
tunneling;
a first spacer layer;
a ferromagnetic free layer that has a magnetic moment in a second
predetermined direction, the second predetermined direction being
different than the first direction, the magnetic moment being free to
rotate relative to the second predetermined direction in response to an
applied magnetic field;
a second spacer layer;
a second enhancement layer capable of enhancing spin polarized resonant
tunneling;
an electrically conductive second ferromagnetic pinned layer;
an electrically conductive second antiferromagnetic pinning layer that has
magnetic spins oriented in the first predetermined direction and by
exchange coupling pins the magnetic moment of the second pinned layer in
the first predetermined direction; and
an electrically conductive second shield layer;
a write head including:
first and second pole piece layers and a write gap layer;
the first and second pole piece layers being separated by the write gap
layer at the ABS and connected at a back gap that is recessed rearwardly
in the head from the ABS;
an insulation stack having at least first and second insulation layers;
at least one coil layer embedded in the insulation stack; and
the insulation stack and the at least one coil layer being located between
the first and second pole piece layers.
23. A magnetic head as claimed in claim 22 wherein the first and second
electrically conductive lead layers are the electrically conductive first
and second shield layers.
24. A magnetic head as claimed in claim 22 wherein the electrically
conductive first and second antiferromagnetic pinning layers have a
thickness from 50 .ANG. to 250 .ANG..
25. A magnetic head as claimed in claim 24 wherein the conductive
antiferromagnetic pinning layers are selected from the group PtMn, MnFe,
NiMn, and IrMn.
26. A magnetic head as claimed in claim 22 wherein the electrically
conductive first and second ferromagnetic pinned layers have a thickness
from 20 .ANG. to 60 .ANG..
27. A magnetic head as claimed in claim 26 wherein the ferromagnetic pinned
layers are selected from the group consisting of Co.sub.90 Fe.sub.10, NiFe
and Co.
28. A magnetic head as claimed in claim 22 wherein the first ferromagnetic
pinned layer is an antiparallel (AP) pinned layer that includes:
a ruthenium (Ru) film layer and first and second ferromagnetic pinned film
layers;
the ruthenium layer being located between the first pinned film layer and
the second pinned film layer;
the first pinned film layer exchange coupled to the first pinning layer so
that a magnetic moment of the first pinned film layer is pinned in the
first predetermined direction; and
the second pinned film layer having a magnetic moment in a third direction,
the third direction being antiparallel to the first direction.
29. A magnetic head as claimed in claim 28 wherein the first and second
pinned film layers are made from cobalt (Co).
30. A magnetic head as claimed in claim 29 wherein the first and second
pinned film layers have a thickness of 25 .ANG. and 20 .ANG. respectively
and the ruthenium layer has a thickness of 8 .ANG..
31. A magnetic head as claimed in claim 28 wherein the second ferromagnetic
pinned layer is an antiparallel (AP) pinned layer that includes:
a ruthenium (Ru) film layer and third and forth ferromagnetic pinned film
layers;
the ruthenium layer being located between the third pinned film layer and
the forth pinned film layer;
the forth pinned film layer exchange coupled to the second pinning layer so
that a magnetic moment of the forth pinned film layer is pinned in the
first predetermined direction; and
the third pinned film layer having a magnetic moment in a third direction,
the third direction being antiparallel to the first direction.
32. A magnetic head as claimed in claim 31 wherein the third and forth
pinned film layers are made from cobalt (Co).
33. A magnetic head as claimed in claim 32 wherein the third and forth
pinned film layers have a thickness of 20 .ANG. and 25 .ANG. respectively
and the ruthenium layer has a thickness of 8 .ANG..
34. A magnetic head as claimed in claim 22 wherein the first and second
enhancement layers have a thickness of 10 .ANG..
35. A magnetic head as claimed in claim 34 wherein the enhancement layers
are made from Cu or Al.
36. A magnetic head as claimed in claim 22 wherein the first and second
spacer layers have a thickness from 10 .ANG. to 30 .ANG..
37. A magnetic head as claimed in claim 36 wherein the spacer layers are
made from aluminum oxide.
38. A magnetic head as claimed in claim 22 wherein the ferromagnetic alloy
free layer has a thickness from 30 .ANG. to 100 .ANG..
39. A magnetic head as claimed in claim 38 wherein the ferromagnetic alloy
free layer is made from NiFe.
40. A magnetic head as claimed in claim 38 wherein the ferromagnetic alloy
free layer is made from a combination of 25% Co.sub.90 Fe.sub.10 and 75%
NiFe.
41. A magnetic head as claimed in claim 22 wherein the first and second
shields are made from Permalloy (Ni.sub.80 Fe.sub.20).
42. A magnetic head as claimed in claim 22 wherein the first predetermined
direction being normal to the ABS.
43. A magnetic disk drive, comprising:
the magnetic head including a combined read head and write head, said
magnetic head having an air bearing surface (ABS);
the read head including:
a resonant enhanced double tunnel junction sensor responsive to applied
magnetic fields; and
first and second electrically conductive lead layers connected to the
double tunnel junction sensor for conducting a tunnel current through the
double tunnel junction sensor; the double tunnel junction sensor
including:
an electrically conductive first shield layer;
an electrically conductive first antiferromagnetic pinning layer that has
magnetic spins oriented in a first predetermined direction;
an electrically conductive first ferromagnetic pinned layer exchange
coupled to the first pinning layer so that a magnetic moment of the first
pinned layer is pinned in the first predetermined direction;
a first enhancement layer capable of enhancing spin polarized resonant
tunneling;
a first spacer layer;
a ferromagnetic free layer that has a magnetic moment in a second
predetermined direction, the second predetermined direction being
different than the first direction, the magnetic moment being free to
rotate relative to the second predetermined direction in response to an
applied magnetic field;
a second spacer layer;
a second enhancement layer capable of enhancing spin polarized resonant
tunneling;
an electrically conductive second ferromagnetic pinned layer;
an electrically conductive second antiferromagnetic pinning layer that has
magnetic spins oriented in the first predetermined direction and by
exchange coupling pins the magnetic moment of the second pinned layer in
the first predetermined direction; and
an electrically conductive second shield layer;
the write head including:
first and second pole piece layers and a write gap layer wherein the first
pole piece layer and the second shield layer are a common layer;
the first and second pole piece layers being separated by the write gap
layer at the ABS and connected at a back gap that is recessed rearwardly
in the head from the ABS;
an insulation stack having at least first and second insulation layers;
at least one coil layer embedded in the insulation stack; and
the insulation stack and the at least one coil layer being located between
the first and second pole piece layers;
a frame;
a magnetic disk rotatably supported on the frame;
a support mounted on the frame for supporting the magnetic head with its
ABS facing the magnetic disk so that the magnetic head is in a transducing
relationship with the magnetic disk;
means for rotating the magnetic disk;
positioning means connected to the support for moving the magnetic head to
multiple positions with respect to said magnetic disk; and
processing means connected to the magnetic head, to the means for rotating
the magnetic disk and to the positioning means for exchanging signals with
the merged magnetic head, for controlling movement of the magnetic disk
and for controlling the position of the magnetic head.
44. A magnetic disk drive as claimed in claim 43 wherein the processing
means is connected to the first and second leads for applying the tunnel
current to the sensor.
45. A magnetic disk drive as claimed in claim 44 wherein the processing
means applies said tunnel current.
46. A magnetic disk drive as claimed in claim 43 wherein the first and
second electrically conductive lead layers are the electrically conductive
first and second shield layers.
47. A magnetic disk drive as claimed in claim 43 wherein the electrically
conductive first and second antiferromagnetic pinning layers have a
thickness from 50 .ANG. to 250 .ANG..
48. A magnetic disk drive as claimed in claim 47 wherein the conductive
antiferromagnetic pinning layers are selected from the group PtMn, MnFe,
NiMn, and IrMn.
49. A magnetic disk drive as claimed in claim 43 wherein the electrically
conductive first and second ferromagnetic pinned layers have a thickness
from 20 .ANG. to 60 .ANG..
50. A magnetic disk drive as claimed in claim 49 wherein the ferromagnetic
pinned layers are selected from the group consisting of Co.sub.90
Fe.sub.10, NiFe and Co.
51. A magnetic disk drive as claimed in claim 43 wherein the first
ferromagnetic pinned layer is an antiparallel (AP) pinned layer that
includes:
a ruthenium (Ru) film layer and first and second ferromagnetic pinned film
layers;
the ruthenium layer being located between the first pinned film layer and
the second pinned film layer;
the first pinned film layer exchange coupled to the first pinning layer so
that a magnetic moment of the first pinned film layer is pinned in the
first predetermined direction; and
the second pinned film layer having a magnetic moment in a third direction,
the third direction being antiparallel to the first direction.
52. A magnetic disk drive as claimed in claim 51 wherein the first and
second pinned film layers are made from cobalt (Co).
53. A magnetic disk drive as claimed in claim 52 wherein the first and
second pinned film layers have a thickness of 25 .ANG. and 20 .ANG.
respectively and the ruthenium layer has a thickness of 8 .ANG..
54. A magnetic disk drive as claimed in claim 51 wherein the second
ferromagnetic pinned layer is an antiparallel (AP) pinned layer that
includes:
a ruthenium (Ru) film layer and third and forth ferromagnetic pinned film
layers;
the ruthenium layer being located between the third pinned film layer and
the forth pinned film layer;
the forth pinned film layer exchange coupled to the second pinning layer so
that a magnetic moment of the forth pinned film layer is pinned in the
first predetermined direction; and
the third pinned film layer having a magnetic moment in a third direction,
the third direction being antiparallel to the first direction.
55. A magnetic disk drive as claimed in claim 54 wherein the third and
forth pinned film layers are made from cobalt (Co).
56. A magnetic disk drive as claimed in claim 55 wherein the third and
forth pinned film layers have a thickness of 20 .ANG. and 25 .ANG.
respectively and the ruthenium layer has a thickness of 8 .ANG..
57. A magnetic disk drive as claimed in claim 43 wherein the first and
second enhancement layers have a thickness of 10 .ANG..
58. A magnetic disk drive as claimed in claim 57 wherein the enhancement
layers are made from Cu or Al.
59. A magnetic disk drive as claimed in claim 43 wherein the first and
second spacer layers have a thickness from 10 .ANG. to 30 .ANG..
60. A magnetic disk drive as claimed in claim 59 wherein the spacer layers
are made from aluminum oxide.
61. A magnetic disk drive as claimed in claim 43 wherein the ferromagnetic
alloy free layer has a thickness from 30 .ANG. to 100 .ANG..
62. A magnetic disk drive as claimed in claim 61 wherein the ferromagnetic
alloy free layer is made from NiFe.
63. A magnetic disk drive as claimed in claim 61 wherein the ferromagnetic
alloy free layer is made from a combination of 25% Co.sub.90 Fe.sub.10 and
75% NiFe.
64. A magnetic disk drive as claimed in claim 43 wherein the first and
second shields are made from Permalloy (Ni.sub.80 Fe.sub.20).
65. A magnetic disk drive as claimed in claim 43 wherein the first
predetermined direction being normal to the ABS.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a double tunnel junction structure used as
a tunnel junction sensor in a magnetic head, and more particularly, to a
double tunnel junction structure having enhancement layers that boost the
magnetoresistance with multiple barriers used to eliminate the effect of
the applied dc bias without reduction in spin polarized tunneling.
2. Description of the Related Art
A read head employing a read sensor may be combined with an inductive write
head to form a combined magnetic head. In a magnetic disk drive, an air
bearing surface (ABS) of the combined magnetic head is supported adjacent
a rotating disk to write information on or read information from the disk.
Information is written to the rotating disk by magnetic fields which
fringe across a gap between the first and second pole pieces of the write
head. In a read mode, the resistance of the read sensor changes
proportionally to the magnitudes of the magnetic fields from the rotating
disk. When a current is conducted through the read sensor, resistance
changes cause potential changes that are detected and processed as
playback signals.
A read sensor is employed by a magnetic head for sensing magnetic fields
from moving magnetic media, such as a magnetic disk or a magnetic tape.
One type of read sensor employs a tunnel junction sensor. The typical
tunnel junction sensor includes a nonmagnetic spacer layer sandwiched
between first and second ferromagnetic layers, commonly called a pinned
layer, and a free layer. The magnetization of the pinned layer is pinned
90.degree. to the magnetization of the free layer and the magnetization of
the free layer is free to respond to external magnetic fields. The
magnetization of the pinned layer is typically pinned by exchange coupling
with an antiferromagnetic pinning layer.
The tunnel junction sensor is based on the phenomenon of spin-polarized
electron tunneling. The typical tunnel junction sensor uses ferromagnetic
metal electrodes, such as NiFe or CoFe, having high coercivity with a
spacer layer that is thin enough that quantum mechanical tunneling occurs
between the ferromagnetic layers (FM/IFM). The tunneling phenomenon is
electron spin dependent, making the magnetic response of the tunnel
junction sensor a function of the relative orientations and spin
polarization of the two ferromagnetic layers. The details of tunnel
junction structures have been described in the commonly assigned U.S. Pat.
No. 5,650,958 to Gallagher et al., which is incorporated by reference
herein.
FIG. 1 shows tunnel magnetoresistance (TMR) as a function of dc bias for a
tunnel junction sensor. At low dc bias, the conduction varies only
slightly with the dc bias. As the dc bias increases, the TMR coefficient
drops noticeably. For example, the application of 300 mV bias across a
tunnel junction structure having a structure comprising
ferromagnetic/insulator/ferromagnetic (FM/I/FM) reduces the TMR by half.
To solve this problem, another type of tunnel junction sensor has been
proposed called a double junction sensor (FM/I/FM/I/FM). FIG. 2 shows a
prior art tunnel junction sensor 200 which includes a first pinning layer
205, a first pinned layer 210, a first spacer layer 215, a free layer 220,
a second spacer layer 225, a second pinned layer 230 and a second pinning
layer 235. The magnetization of the outer two FM pinned layers are
parallel while the magnetization of the internal FM free layer is either
parallel or antiparallel. Modeling has shown that the double tunnel
junction behaves differently than the traditional single tunnel junction
by eliminating the effect of dc bias. FIG. 3 shows the TMR as a function
of the dc bias for a double junction tunnel junction sensor.
While it appears that the multiple barriers have been shown to
significantly eliminate the effect of dc bias, the double tunnel junction
has drawbacks. For the spin polarized resonant tunneling phenomenon to
work, the layers of the double tunnel junction must be made very thin.
While it is desired to have thin layers, too thin a layer is detrimental
to the device. For example, the center FM layer (traditionally the free
layer) for the prior art is between 10 and 20 .ANG.. With a layer this
thin, the ferromagnetic free layer becomes saturated easily from external
magnetic fields. Once saturated, the double tunnel junction sensor does
not get the full benefit of the ferromagnetic free layer, the signals get
clipped. It is preferable that the free layer never be saturated.
From the above discussion it becomes apparent that what is needed is a
double tunnel junction sensor that provides the benefits of improved spin
polarized tunneling and minimizing dc bias effects while also providing a
device in which the internal layers are not saturated by an external
magnetic field.
SUMMARY OF THE INVENTION
The present invention is directed toward an enhanced double tunnel junction
structure that has enhancemnt layers causing resonant tunneling which
boosts the magnetoresistance (MR), achieving higher tunnel
magnetoresistance (TMR) for the structure. This is accomplished by using
enhancement layers that create a quantum well between the enhancement
layer and the pinned layer. By doing this, the tunneling constraints on
the free layer are decoupled, allowing the free layer to be made thicker
(>20 .ANG.) and reducing or eliminating saturation from an external
magnetic source.
In one embodiment, the resonant enhanced double tunnel junction sensor
includes a first shield, a first pinning layer, a first pinned layer, a
first enhancement layer, a first spacer layer, a free layer, a second
spacer layer, a second enhancement layer, a second pinned layer, a second
pinning layer and a second shield layer. In the preferred embodiment, the
enhancement layer is made form copper (Cu). In another embodiment, the
free layer is a multi-layered material having 75% NiFe and 25%Co.sub.90
Fe.sub.10.
In the preferred embodiment, the magnetic moment of the first and second
pinned layers are pinned by interfacial exchange with the magnetic spins
of the first and second pinning layers in a downward direction,
perpendicular to the ABS, while the magnetic moment of the free layer is
perpendicular to the magnetic moment of the first and second pinned layers
(i.e., the moment direction being parallel to the ABS). In use, a
tunneling current I.sub.T, using spin dependent electron tunneling, flows
through the enhanced double tunnel junction sensor, using the first and
second shield layers as leads. The amount of current I.sub.T that flows
through is dependent on the relative magnetic moment directions between
the first and second pinned layers and the free layer. In prior art double
tunnel junctions, the free layer must be thin to perform properly and is
prone to become saturated quickly from the external magnetic field. To
solve this problem, the present invention adds enhancement layers of
copper (Cu) to boost the magnetoresistivity (MR) of the sensor. The copper
enhancement layers increase the spin polarized resonant tunneling, giving
the structure a high TMR. With the higher TMR, the free layer may be made
thicker and not saturate as easily. As the enhanced double tunnel junction
sensor is positioned over the magnetic disk, the external magnetic fields
sensed from the rotating disk moves the direction of magnetic moment of
the free layer up or down, changing the resistance through the tunnel
junction sensor. As the tunnel current I.sub.T is conducted through the
sensor, the increase and decrease of electron tunneling (i.e., increase
and decrease in resistance) are manifested as potential changes. These
potential changes are then processed as readback signals by the processing
circuitry.
Another embodiment of the present invention is an antiparallel (AP)
resonant enhanced double tunnel junction sensor. This AP double tunnel
junction sensor is similar to the double tunnel junction sensor described
above but utilizes first and second AP pinned layers and in place of the
first and second pinned layers. The AP pinned layer consists of a spacer
made of ruthenium (Ru) between pinned film layers, preferably made of
cobalt (Co). Because of the antiparallel features of the AP layers due to
the Ru spacer layer, the magnetic moment of the one pinned film is
antiparallel to magnetic moment of the other pinned film, which increases
the effect of the sensor when the magnetic moment of the free layer
rotates. In other embodiments, a combinations of pinned and AP pinned
layers are used.
Other objects and advantages of the present invention will become apparent
upon reading the following description taken together with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of tunnel magnetoresistance (TMR) vs. dc bias for a
traditional tunnel junction structure (FM/I/FM);
FIG. 2 is an air bearing surface (ABS) illustration of a prior art double
tunnel junction structure (FM/I/FM/I/FM);
FIG. 3 is a graph of TMR vs. dc bias for the structure of FIG. 2;
FIG. 4 is a plan view of an exemplary magnetic disk drive;
FIG. 5 is an end view of a slider with a magnetic head as seen in plane
5--5 of FIG. 4;
FIG. 6 is an elevation view of the magnetic disk drive wherein multiple
disks and magnetic heads are employed in a housing;
FIG. 7 is an isometric illustration of an exemplary suspension system for
supporting the slider and magnetic head;
FIG. 8 is an ABS view of the slider taken along in plane 8--8 of FIG. 5;
FIG. 9 a side view of a front portion of the magnetic head as seen in plane
9--9 of FIG. 5;
FIG. 10 is a partial ABS view of the slider taken along plane 10--10 of
FIG. 9 to show the read and write elements of the magnetic head;
FIG. 11 is a view taken along plane 11--11 of FIG. 9 with all material
above the coil layer and its leads removed;
FIG. 12 is an air bearing surface (ABS) illustration of one embodiment of
the sensor of the present invention;
FIG. 13 is an air bearing surface (ABS) illustration of another embodiment
of the sensor of the present invention; and
FIG. 14 is an illustration of alternate embodiments using multi-layer
construction for the free layer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Magnetic Disk Drive
Referring now to the drawings wherein like reference numerals designate
like or similar parts throughout the several views, FIGS. 4-6 illustrate a
magnetic disk drive 30. The drive 30 includes a spindle 32 that supports
and rotates a magnetic disk 34. The spindle 32 is rotated by a motor 36
that is controlled by a motor controller 38. A combined read and write
magnetic head (merged MR head) 40 is mounted on a slider 42 that is
supported by a suspension 44 and actuator arm 46. A plurality of disks,
sliders and suspensions may be employed in a large capacity direct access
storage device (DASD) as shown in FIG. 3. The suspension 44 and actuator
arm 46 position the slider 42 so that the magnetic head 40 is in a
transducing relationship with a surface of the magnetic disk 34. When the
disk 34 is rotated by the motor 36 the slider is supported on a thin
(typically, 0.05 .mu.m) cushion of air (air bearing) between the surface
of the disk 34 and the air bearing surface (ABS) 48. The magnetic head 40
may then be employed for writing information to multiple circular tracks
on the surface of the disk 34, as well as for reading information
therefrom. Processing circuitry 50 exchanges signals, representing such
information, with the head 40, provides motor drive signals for rotating
the magnetic disk 34, and provides control signals for moving the slider
to various circular tracks on the disk. FIG. 7 shows the mounting of the
slider 42 to the suspension 44, which will be described hereinafter. The
components described hereinabove may be mounted on a frame 54 of a housing
55, as shown in FIG. 6.
FIG. 8 is an ABS view of the slider 42 and the magnetic head 40. The slider
has a center rail 56 that supports the magnetic head 40, and side rails 58
and 60. The rails 56, 58 and 60 extend from a cross rail 62. With respect
to rotation of the magnetic disk 34, the cross rail 62 is at a leading
edge 64 of the slider and the magnetic head 40 is at a trailing edge 66 of
the slider.
FIG. 9 is a cross-sectional elevation side view of a front portion of the
merged MR head 40, which includes a write head portion 70 and a read head
portion 72, the read head portion employing an enhanced double tunnel
junction sensor 74 of the present invention. FIG. 10 is an ABS view of
FIG. 9. The sensor 74 and insulating gap layer 75 are sandwiched between
first and second shield layers 80 and 82. The insulating gap layer 75
insulates the shields from each other and may be made from aluminum oxide,
aluminum nitride or silicone dioxide. In response to external magnetic
fields from the rotating disk, the resistance of the tunnel junction
sensor 74 changes. To determine the resistance, a tunneling current
I.sub.T is used. The first and shield layers 80 and 82 are employed as
leads. The current flows through all the layers of the tunnel junction
between the leads (i.e., first and second shields). As the free layer
rotates in response to the magnetic field from the disk, the resistance of
the tunnel junction structure changes, altering the current through the
structure. These resistance changes are manifested as potential changes.
These potential changes are then processed as readback signals by the
processing circuitry 50 shown in FIG. 6.
The write head portion of the merged MR head includes a coil layer 84
sandwiched between first and second insulation layers 86 and 88. A third
insulation layer 90 may be employed for planarizing the head to eliminate
ripples in the second insulation layer caused by the coil layer 84. The
first, second and third insulation layers are referred to in the art as an
"insulation stack". The coil layer 84 and the first, second and third
insulation layers 86, 88 and 90 are sandwiched between first and second
pole piece layers 92 and 94. The first and second pole piece layers 92 and
94 are magnetically coupled at a back gap 96 and have first and second
pole tips 98 and 100 which are separated by a write gap layer 102 at the
ABS. As shown in FIGS. 5 and 7, first and second solder connections 104
and 116 connect leads from the tunnel junction sensor 74 to leads 112 and
124 on the suspension 44, and third and fourth solder connections 118 and
116 connect leads 120 and 122 from the coil 84 to leads 126 and 114 on the
suspension.
Present Invention
The present invention is directed toward an enhanced double tunnel junction
structure that has enhancement layers. These enhancement layers boost the
MR to achieve a higher TMR for the structure. This is accomplished by
using enhancement layers to create a quantum well between the enhancement
layer and the pinned layer causing resonance which enhances the tunneling
electrons. By doing this, the tunneling constraints on the free layer are
decoupled, allowing the free layer to be made thicker (>20 .ANG.) and
reducing or eliminating saturation of the free layer from an external
magnetic source. FIG. 12 shows one embodiment of the present invention of
a resonant enhanced double tunnel junction sensor 300 which includes a
first shield 80, a first protection layer 302 (if necessary), a seed layer
303 (if necessary), a first pinning layer 305, an interface layer 306, a
first pinned layer 310, a first enhancement layer 314, a first spacer
layer 315, a free layer 320, a second spacer layer 325, a second
enhancement layer 326, a second pinned layer 330, an interface layer 332,
a second pinning layer 335, a second protection layer 337 (if necessary)
and a second shield layer 82. The first and second shields, 80 and 82, are
made from a conductive material, such as Permalloy, which is Ni.sub.80
Fe.sub.20. The first and second protection layers 302 and 337 are made of
tantalum (Ta), having a thickness of 10-100 .ANG., with a preferred
thickness of 50 .ANG.. The protection layers are used to protect the
sensor from damage during subsequent processing and to isolate the sensor
from the shields. The protection layers are also known as de-coupling
layers. Depending on the processing, the protection layers may not be
necessary. The seed layer 303 is made of nickel iron (NiFe) with a
thickness of 10-20 .ANG.. The seed layer is used to control the grain
size, texture and crystal structure. In certain instances, the seed layer
may not be necessary. The first and second pinning layers, 305 and 335,
are preferably made of an antiferromagnetic material, such as platinum
manganese (PtMn), with a thickness range of 50-250 .ANG., preferably 100
.ANG.. In the preferred embodiment, the magnetic spins of the first and
second pinning layers are parallel with each other. Optionally, the
pinning layers may be made of manganese iron (MnFe), nickel manganese
(NiMn) or iridium manganese (IrMn). The interface layers 306 and 332 are
made of nickel iron (NiFe), with a thickness of 10-30 .ANG., preferably 20
.ANG., and are used between the pinning layers and the pinned layers to
enhance exchange coupling. The reason for the interface layers is that the
pinning layer material has a stronger exchange coupling with the NiFe
material than the pinned layer material. The first and second pinned
layers, 310 and 330, are preferably made from a ferromagnetic material,
such as cobalt iron (Co.sub.90 Fe.sub.10), with a thickness of 20-60
.ANG., preferably 40 .ANG.. Optionally, the pinned layers may be made from
nickel iron (NiFe) or Cobalt (Co). The first pinned layer 310 is exchange
coupled to the first pinning layer 305 and the second pinned layer 330 is
exchange coupled to the second pinning layer 335. In the preferred
embodiment, the magnetic moment of the first and second pinned layers, 310
and 330, are parallel. The first and second enhancement layers, 314 and
326, are preferably made of copper (Cu), with a thickness of 10 .ANG..
Optionally the enhancement layers may be made from aluminum (Al) or any
other conductive material that increases spin polarized resonant
tunneling. The first and second spacer layers, 315 and 325, are preferably
made of aluminum oxide, with a thickness of 10-30 .ANG., preferably 20
.ANG.. The free layer 210 is made from nickel iron (NiFe), with a
thickness of 30-100 .ANG., preferably 40 .ANG.. Optional embodiments of
the free layer use a multi-layer construction of material and thicknesses
having 75% NiFe and 25%Co.sub.90 Fe.sub.10. FIG. 14 shows examples of the
multi-layer free layers. Multi-layer free layer 360 is comprised of a 5
.ANG. cobalt iron (Co.sub.90 Fe.sub.10) first layer 361, a 30 .ANG. nickel
iron (NiFe) second layer 362 and a 5 .ANG. cobalt iron (Co.sub.90
Fe.sub.10) third layer 363. Multi-layer free layer 365 is comprised of a
10 .ANG. cobalt iron (Co.sub.90 Fe.sub.10) first layer 366 and a 30 .ANG.
nickel iron (NiFe) second layer 367. While the above description presents
material options for the various layers, it is understood that equivalent
materials may be substituted and fall within the scope of the present
invention.
In the preferred embodiment, the magnetic moment of the first and second
pinned layers, 310 and 330, are pinned in a downward direction
perpendicular to the ABS, due to interfacial exchange with the magnetic
spins of the adjacent first and second pinning layers, 305 and 335. The
magnetic moment of the free layer 320 is in a different direction than the
pinned layers, such as a canted relationship, preferably perpendicular to
the magnetic moment of the first and second pinned layers, 310 and 330
(i.e., the moment direction being parallel to the ABS). In use, a
tunneling current I.sub.T, using spin dependent electron tunneling, flows
through the tunnel junction sensor 300, using the first and second shield
layers, 80 and 82, used as leads. The amount of current I.sub.T that flows
through is dependent on the relative magnetic moment directions between
the first and second pinned layers 310 and 330 and the free layer 320. As
the tunnel junction sensor 300 is positioned over the magnetic disk 34,
the external magnetic fields sensed from the rotating disk 34 moves the
direction of magnetic moment of the free layer 320 up or down, changing
the resistance through the enhanced double tunnel junction sensor 300. The
use of the resonant enhancement layers, 314 and 326, further enhance the
change in resistance (.DELTA.R/R) of the enhanced double tunnel junction
sensor 300. As the magnetic moment of the free layer 320 rotates up from
the ABS (i.e., going toward the opposite direction of the magnetic moment
of the first and second pinned layers, 310 and 330), the amount of
electron tunneling decreases (i.e., the resistance increases). As the
magnetic moment of the free layer 320 rotates down toward the ABS (i.e.,
going toward the same direction as the magnetic moment of the first and
second pinned layers, 310 and 330), the amount of electron tunneling
increases (i.e., the resistance decreases). As the tunnel current I.sub.T
is conducted through the sensor 300, the increase and decrease of electron
tunneling (i.e., increase and decrease in resistance) are manifested as
potential changes. These potential changes are then processed as readback
signals by the processing circuitry shown in FIG. 6. To boost the
magnetoresistivity (MR) of the sensor 300, the copper (Cu) enhancement
layers are positioned next to the spacer layers of aluminum oxide. The
copper enhancement layers increase the spin polarized resonant tunneling,
giving the structure a high TMR. With the higher TMR, the free layer may
be made thicker and not saturate as easily.
FIG. 13 is another embodiment of the present invention showing an
antiparallel (AP) resonant enhanced double tunnel junction sensor 350.
This sensor double tunnel junction sensor 350 is similar to the double
tunnel junction sensor 300 described above but utilizes a first and second
AP pinned layers, 340 and 345, in place of the first and second pinned
layers, 310 and 330. The first AP pinned layer 340 consists of a spacer
342, made of ruthenium (Ru), with a thickness of 8 .ANG., located between
a first pinned film 341 and a second pinned film 343, preferably made of
cobalt (Co), with a thickness of 25 .ANG. and 20 .ANG. respectively. The
second AP pinned layer 345 consists of a spacer 347, made of ruthenium
(Ru), with a thickness of 8 .ANG., located between a third pinned film 346
and a forth pinned film 348, preferably made of cobalt (Co), with a
thickness of 20 .ANG. and 25 .ANG. respectively. Optionally, the pinned
films may be made of nickel iron (NiFe). Because of the antiparallel
features of the AP pinned layer 340 due to the Ru spacer layer, the
magnetic moment of the first pinned film 341 is in the same direction as
the magnetic spins of the first pinning layer 305 by interfacial exchange,
while the magnetic moment of the second pinned film 343 is in an
antiparallel direction. Similarly, the direction of the magnetic moment of
the forth pinned layer 348 is pinned by interfacial exchange with the
adjacent second pinning layer 335, with the preferred embodiment in a
downward direction perpendicular to the ABS. Because of the antiparallel
features of the AP pinned layer 345 due to the spacer layer 347, the
magnetic moment of the third pinned film 346 is antiparallel to magnetic
moment of the forth pinned film 348. Having the magnetic moments of the
second and third pinned film layers, 343 and 346, antiparallel to the
magnetic moments of the first and forth pinned film layers, 341 and 348,
increases effect of the sensor when the magnetic moment of the free layer
320 rotates. In another embodiment, a combination of a pinned layer (310
or 330) and an AP pinned layer (340 or 345) is used. In this embodiment,
the pinned layer 310 is used with AP pinned layer 345 or the pinned layer
330 is used with AP pinned layer 340. In still another embodiment, the
multi-layer free layer 360 or 365 (see FIG. 14) is used with AP pinned
layers 340 and 345 or used with the combination of the pinned layer (310
or 330) and AP pinned layer (340 or 345), as described above.
Clearly, other embodiments and modifications of this invention will occur
readily to those of ordinary skill in the art in view of these teachings.
While the description of the enhanced double tunnel junction sensor is
described in relation to a magnetic disk drive read/write head, it should
understood that in other applications, the enhanced double tunnel junction
sensor may be used alone or in combination with other devices. Therefore,
the disclosed invention is to be considered merely illustrative and
limited in scope only as specified in the appended claims.
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